In precast concrete or reinforced concrete components, or in road surface or runway renewals, there are increasing demands for a high early strength, in order that the precast components can be demolded, transported, stacked or prestressed after only a few hours, or the road surfaces or runways can be traveled upon. In order to achieve this in practice, high-performance concrete formulations, such as low w/c ratio or high cement content, are employed, as are heat or steam treatments. These treatments involve a great deal of energy, and so this treatment is increasingly being abandoned owing to rising energy costs, considerable capital costs and problems with durability and exposed concrete, and other ways of accelerating the curing process are being sought.
Accelerating additives have to date not constituted a satisfactory alternative to heat or steam treatment. There are many known substances which accelerate the setting and curing of concrete. Examples in use are strongly alkaline substances such as alkali metal hydroxides, alkali metal carbonates, alkali metal silicates, alkali metal aluminates and alkaline earth metal chlorides. In the case of the strongly alkaline substances, however, undesired hazards for the user, such as chemical burns, can occur, and they reduce the final strength and the durability of the concrete.
EP 0076927 B1 and EP 0 946 451 B1 disclose alkali-free solidification accelerators for hydraulic binders. To accelerate the solidification and curing of a hydraulic binder, such as cement, lime, hydraulic lime and gypsum, and mortar and concrete produced therefrom, an alkali-free solidification and curing accelerator is added, the accelerator including aluminum hydroxide and optionally aluminum salts and organic carboxylic acids.
Although such known accelerators accelerate the setting and curing of hydraulically setting systems, they are expensive, the use thereof can be restricted owing to inadequate durability and insufficient efficacy, and they reduce the processing time but have an adverse effect on the final strengths of the concrete. Such setting and curing accelerators are additionally a relatively low early strength in the first hours and days, and possess inadequate solution stability.
The systems known at present, where the hydration of a concrete is accelerated by the addition of a setting accelerator, mostly relate to spray concrete. With known methods of controlling hydration, the cement mixtures set very rapidly after addition of the accelerator. This can be desirable, especially in the case of use as spray concrete. However, such known systems are unsuitable when the cement mixture has to be processed further after activation or when the processed concrete has to be stressed after a short time. In the case of the known systems for spray concrete application, however, there is no further processability after activation.
The present disclosure is directed to an admixture which can accelerate the setting and curing process of compositions containing hydraulic binders, which not only enables use in spray concrete, but with which it is possible to produce a quick-curing mortar or concrete composition which has a high early strength and good processing properties, and thus enables early demolding or early stress.